Directional and non-directional boring apparatus for making holes through soil are well known. A directional borer generally includes a series of drill rods joined end to end to form a drill string. The drill string is pushed or pulled through the soil by means of a powerful hydraulic device such as a hydraulic cylinder. A spade, bit or chisel configured for boring is disposed at the end of the drill string, which may include an ejection nozzle for water to assist in boring. In general, the direction of boring is controlled by selectively rotating a boring head having an angled face. During rotation, the borer continues straight, whereas when pushed without rotation the boring head moves in the favored direction. See Malzahn U.S. Pat. Nos. 4,945,999 and 5,070,948, and Cherrington U.S. Pat. No. 4,697,775 (RE 33,793). The drill string may be pushed and rotated at the same time as described in Dunn U.S. Pat. No. 4,953,633 and Deken et al. U.S. Pat. No. 5,242,026.
In one variation of the traditional boring system, a series of drill string rods is used in combination with a percussion tool mounted at the end of the series of rods. The rods can supply a steady pushing force to the impact tool and the interior of the rods can be used to supply the pneumatic borer with compressed air. See McDonald et al. U.S. Pat. No. 4,694,913. This system has, however, found limited application commercially, perhaps because the drill string tends to buckle when used for pushing if the bore hole is substantially wider than the diameter of the drill string.
A variety of systems are now known for the installation of underground pipes, particularly for the replacement of an existing deteriorated pipe. In one widely practiced method, a pneumatic impact boring tool is sent through the existing pipeline such that the head of the tool, which may be provided with blades that apply intense local pressure to the existing pipe, fractures or splits the existing pipe. See, for example, Streatfield et al. U.S. Pat. Nos. 4,720,211, 4,738,565 and 4,505,302. A replacement pipe, typically made of plastic such as HDPE, can be drawn along behind the boring tool. This process has proven effective commercially because it bursts the old pipe and replaces it with a new pipe at the same time. However, the system relies on a pneumatic impact tool, which in turn requires an air compressor. Exhaust from the impact tool is vented into the interior of the replacement pipe, which is unacceptable for certain types of pipe installations, such as gas and water lines.
Directional borers are less effective for pipe bursting, especially for hard to burst pipes like cast iron, because the steady pushing force of the drill string lacks the impact power of a pneumatic impact boring tool. Thus, in some instances, a directional borer or winch is used to pull a pneumatic impact tool through an existing pipeline in order to burst the existing pipe and pull in the replacement pipe. These alternatives are effective but require considerable equipment and manual labor. A need remains for a boring system that can avoid the need for a pneumatic impact tool and still provide cyclic impacts suitable for pipe bursting operations, rock breaking, and the like.